The most simple solution to this is to uninstall Messenger Plus! while making sure you choose not to keep your current settings. After that you can install again and the default settings (so no password) will apply.
If you are comfortable with editing the registry, you can also check out
CookieRevised's reply to Preferences Password. However, this way of resetting the password is not recommended if you are not an experienced computer user. In that case simply uninstall and reinstall as mentioned above
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